1. Field of the Invention
The present invention generally relates to optical pickup devices used in optical disk recording and/or reproducing apparatuses, and more particularly to an optical pickup device enabling recording and/or reproducing with two or more types of recording media.
2. Description of Prior Art
An optical writing and/or reading system carries out recording and/or reproducing of information such as video, audio or other data to/from a recording medium. In such system, a semiconductor laser is used for generating a laser beam, and an objective lens is used for converging the laser beam and forming a focused spot on the recording medium. The recording density of the recording medium is determined by the size of the focused spot. In general, the size of the focused spot (S) is proportional to the wavelength (λ), and inversely proportional to the numerical aperture (NA), as expressed by formula (1):S∝λ/NA  (1)
Therefore, to increase the recording density, the size of the spot being focused on the optical disk must be reduced. To reduce the spot size, as can be inferred from formula (1), the wavelength (λ) of the laser beam must be reduced and/or the numerical aperture (NA) of the objective lens must be increased. This has been demonstrated by the ongoing development of optical recording media. For example, the wavelength of read beams for compact disks (CDs) is about 780 nm, the wavelength of read beams for digital versatile disks (DVDs) is about 650 nm, and the wavelength of read beams for high-definition DVDs (HD-DVDs) is about 405 nm. Furthermore, the numerical aperture for CDs is 0.45, the numerical aperture for DVDs is 0.6, and the numerical aperture for HD-DVDs is 0.65-0.8.
On the other hand, coma aberration, which occurs due to a tilting of the optical disk, is associated with the tilt angle of the disk, the refractive index of the disk substrate, the thickness of the disk substrate, and the numerical aperture of the objective lens. To ensure an acceptable level of coma aberration with respect to the tilt of a disk for high-density recording, the thickness of the disk substrate is in general reduced accordingly. For example, CDs have a thickness of 1.2 mm, and DVDs have a thickness of 0.6 mm. Further, the thickness of many HD-DVDs is 0.6 mm or less.
In an apparatus for high-density recording onto or playing from a medium such as a HD-DVD, a primary consideration is the compatibility of the apparatus with existing disks including CDs and DVDs. Conventionally, there are two kinds of optical writing and/or reading systems that are used in multi-compatible home entertainment players. In the first kind of optical writing and/or reading system, an independent optical system is provided therein for each type of disk. That is, generally, the optical writing and/or reading system has at least three light sources and three objective lenses for these disks. This kind of writing and/or reading system needs too many optical elements, and is unduly large and costly. In the second kind of writing and/or reading system, there are some common optical elements that function for both existing disks and for HD-DVDs; for example, a common objective lens. This kind of writing and/or reading system reduces the total number of optical elements and simplifies the overall configuration. However, the optical performance of the optical pickup head is limited. In respect of the common objective lens, chromatic aberration occurs because each kind of disk operates according to different wavelengths. Further, spherical aberration occurs because the disks have different thicknesses.